This invention relates to a heating unit for heating a photosensitive planographic printing original plate comprising a photosensitive layer placed on a metal support while transporting the plate after exposure and in particular to a heating unit applicable as a unit for heat developing of a printing original plate.
A method of executing image exposure of a photosensitive material containing silver halide, a reducing agent, and a polymerizable compound and developing the silver halide, thereby polymerizing the polymerizable compound like an image and forming a polymer image is described in Japanese Patent Examined Publication No. Hei. 3-12307 and Japanese Patent Examined Publication No. Hei. 3-12308 (U.S. Pat. No. 4,629,676 and EP No. 0174634).
In this method, polymerization is started by oxidant radical of the reducing agent reducing the silver halide (the radical may be a radical produced by decomposition of the oxidant of the reducing agent; it will be hereinafter referred to simply as oxidant radical). That is, the photosensitive material is heated, the silver halide is developed, and a cure image of the polymerizable compound is formed as execution of heat developing.
The described image formation method can also be applied to manufacturing of a printing plate.
Photosensitive materials preferred for manufacturing a printing plate are described in Japanese Patent Unexamined Publication No. Sho. 64-17047 (U.S. Pat. No. 4,985,339 and EP. No. 0298522), Japanese Patent Unexamined Publication No. Hei. 5-249667 (U.S. Pat. No. 5,122,443 and EP No. 0426192), Japanese Patent Unexamined Publication No. Hei. 4-191856 (U.S. Pat. No. 5,290,659), Japanese Patent Unexamined Publication No. Hei. 5-142775, Japanese Patent Unexamined Publication No. Hei. 6-27652, and Japanese Patent Unexamined Publication No. Hei. 5-107764. The photosensitive materials described in these publications are characterized by the fact that cure reaction using silver halide as an optical sensor is used; at least three layers of a curable layer containing a polymerizable compound or a cross-linkable polymer, a photosensitive layer containing silver halide, and an overcoat layer containing a base precursor are deposited in order on a support and a reducing agent is contained in any layer (the photosensitive material will be hereinafer referred to as silver trigger polymerization type photosensitive material). In the printing plate manufacturing method, the polymerizable compound or cross-linkable polymer is cured by the above-mentioned heat developing, then an eluate is used to remove uncured portions and the remaining cure image is used as a printing plate image.
The printing plate uses a metal plate (generally, an aluminum plate) as a support.
To thus form an image on a printing original plate comprising the silver trigger polymerization type photosensitive material placed on a metal support, a heat treatment unit is required.
Such heat treatment is roughly classified into a non-contact heating method of applying a heat ray from a heater to a printing original plate and a contact heating method wherein a printing original plate comes in contact with a heater, as described in Japanese Patent Unexamined Publication No. Hei. 4-240643.
A heating unit adopting the contact heating method is adapted to sandwich a printing original plate 114 between an endless belt 138 and a heating plate 130 and as the endless belt. 138 moves, to transport the printing original plate 114 while bringing the printing original plate 114 into sliding contact with the heating plate 130 and heat the printing original plate 114 by heat of the heating plate 130, for example, as shown in FIG. 8. The surface of the heating plate 130 is bent like a circular arc and the printing original plate 114 is made sufficiently thin to curve along the bend. The endless belt 138 is given tension for pressing the printing original plate 114 against the heating plate 130. Transport rollers 122 and 124 are disposed ahead and behind the heating plate 130 in the transport direction for transporting the printing original plate 114 into nip between the heating plate 130 and the endless belt 138 and transporting the printing original plate 114 out of the nip as indicated by arrows A.
The printing original plate 114 transported into the nip between the heating plate 130 and the endless belt 138 by means of the transport rollers 122 and 124 is transported while it is brought into sliding contact with the heating plate 130 as the endless belt 138 is turned as indicated by arrow D, and is heated by heat of the heating plate 130. At this time, the endless belt 138 is given tension by a bend part 130B of the heating plate surface, whereby the printing original plate 114 is pressed against the heating plate side by the endless belt 138, comes in intimate contact with the heating plate, and is heated entirely. Temperature ranging from about 70.degree. C. to 200.degree. C. is used as the heating temperature of the silver trigger polymerization type photosensitive material.
By the way, since the printing original plate 114 has the support made of a metal plate, thermal expansion occurs upon heating. Since the printing original plate 114 is pressed against the heating plate 130 by the endless belt 138 and comes in intimate contact with the heating plate 130, a shift at either end is limited and extension caused by the thermal expansion goes from the end margin to the inside. Normally, the printing plate becomes deformed so as to protrude at the center roughly over the full length. Since the printing original plate 114 is warped due to the thermal expansion, the contact between the printing original plate 114 and the heating plate 130 becomes uneven and the heat developing efficiency is degraded in the incomplete contact part. Resultantly, the polymerization cure image in this part suffers a loss or the cure degree is degraded and the image strength lowers; if the printing original plate 114 is used as a printing plate, printing resistance is degraded partially. For example, if an attempt is made to form such minute dots with the number of halftone lines of 200 and the halftone dot area percentage of 2%, the heat developing efficiency in the incomplete contact part is degraded and insufficient cure occurs. Thus, minute halftone dots are formed in small size as compared with those formed in the complete contact part and tone differs partially; uniformity of the printing original plate 114 for use as a printing plate is not satisfied.
On the other hand, a heating unit having a structure as shown in FIG. 9 is available with the above-mentioned non-contact heating method. This heating unit comprises a heater 142 and a guide plate 143 for guiding a printing original plate in a heat insulation case 141, the surroundings of which 141 are covered with a case 144. This case 144 contains transport-in rollers 145 for transporting a print original plate inserted from the arrow A direction into the heating unit and transport-out rollers 146.
In the heating unit, the print original plate inserted from the arrow A direction runs below the heater 142 at constant speed and is heated, then is sandwiched between the transport-out rollers 146 and is transported to the next step.
For heating of the heater 142, the temperatures of parts of the heater are controlled based on the temperature detected by a temperature sensor provided in the heater 142.
The problem involved in the contact heating method, namely, a heating failure caused by deformation such that a printing original plate protrudes at the center roughly over the full length does not occur in the non-contact heating method.
However, in the non-contact heating method, the leading and trailing end arrival temperatures vary as shown in FIG. 1C in response to the parts of a printing original plate as the result of measuring the temperature of the printing original plate, because a temperature gradient occurs between a heated portion and an unheated portion and energy moves, whereby the temperature of the leading end portion lowers and that of the trailing end portion rises. Therefore, the printing original plate is not heated uniformly.
Further, the time stability of the temperature in the heat insulation case 141, in other words, the atmospheric temperature in the heating part is not good, causing uneven heating.